Acoustic guitars are often amplified during performances. Typically this amplification is accomplished by holding the guitar adjacent a stationary microphone during the performance, or actually mounting a small microphone on the face of the guitar near the "sound hole."
Another approach is to mount a microphone in the sound hole itself. An example of an "in-hole" pick-up is the Pro-Mag brand pick-up, sold by Dean Markley.
Feedback is a problem when an acoustic guitar is amplified using any of these microphones. Specifically, "tone rollover," a feedback phenomena, is common at frequencies of 160 mH, 400 mH, and 1K. This feedback produces an undesirable buzz or hum, and is unpredictable and common when acoustical instruments are amplified.
Certain devices have been described which can reduce or eliminate this feedback. U.S. Pat. No. 4,394,830 to Damiano describes a plug to cover the sound hole of an acoustic guitar. The plug completely covers the hole. In one embodiment, a magnetic pick-up is integrated into the plug.
The plug of the '830 patent cannot, however, be used with in-hole amplification devices which are designed to fit into the sound hole, such as the Dean Markley Pro-Mag device. Thus, there is a need for a feed back controlling device which would permit the use of an in-hole microphone.